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"During the titration of a weak polyprotic acid with a strong base, the first stoichiometric equivalence point occurs when the moles of added base are equal to the moles of the initial acid species. Prior to the equivalence point, a buffering region forms in which both the weak acid and its conjugate base are present and the pH is determined by the pKa and the ratio of the acid and base species concentrations. When the concentrations of the acid and base species are equal, pH = pKa."

The bolded statements seem like they're saying the same thing. What is the difference between the buffering region and the equivalence point?

Isn't the buffering region when [HA] = [A-], so that in the HH equation, pH = pka? The first bolded statement makes it sound like the first equivalence point is also when [HA] = [A-].

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  • $\begingroup$ You're absolutely correct. The word base was used in two different ways which is confusing. $\endgroup$ – MaxW Mar 15 '19 at 20:48
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This is a matter of definitions which you must learn. Let's say that the titrant is NaOH and that we have a monoprotic acid.

Equivalence point - The equivalence point occurs when the moles of added base are equal to the moles of the initial acid species.

Notice that this doesn't infer anything about the pH of the solution.

Now for a polyprotic acid there are multiple equivalence points. The first equivalence point occurs when the moles of added base are equal to the moles of the initial acid species. The second occurs when twice as many moles of added base are added as compared to the initial acid species. The third three times as much base as acid and so on.

Again notice that this doesn't infer anything about the pH of the solution at the multiple equivalence points.

When the concentrations of the acid and base species are equal, pH = pKa."

This is a poor statement. Let's let the acid be $\ce{HA}$ so the conjugate base is $\ce{A^-}$. The statement would have been much better expressed as:

When the concentrations of the acid and its conjugate base are equal, pH = pKa."

Now to wrap this up, imagine doing this experiment. We start off with $\ce{HA}$ in solution. As we add more base we get to the pH where $\ce{[HA] = [A^-]}$ and where (1) there is maximum buffering capacity, and (2) the pH = pKa of the acid. We now add more base and we finally get to the equivalence point.

PS - For titrations you should also know the endpoint which is where the chemist gets some sort of signal to stop the titration.

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I started reading about chemistry 2 months ago, so take this with a grain of salt. I also learned this in norwegian, so terms migt be different..

As far as i know, the equivalence point is when there is no longer any of the reactant, because it has all reacted with the added base/acid. Whereas the buffering/halfway titration point is when, as they say, there are equal amounts of H- as HA, making the solution a buffer.

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  • $\begingroup$ That is not the correct definition for the equivalence point. $\endgroup$ – MaxW Mar 15 '19 at 20:43
  • $\begingroup$ I know the definition is what you stated above, but is it not also true that at the equivalence point, all the initial acid has reacted with added base(which naturally have to be the same amount)? $\endgroup$ – Quantonium Mar 16 '19 at 8:57
  • $\begingroup$ Not necessarily. At the equivalence point we'd assume that a strong acid and a strong base would completely react. However that wouldn't be true for weak acid/strong base, strong acid/weak base, or weak acid/weak base. $\endgroup$ – MaxW Mar 16 '19 at 14:44
  • $\begingroup$ I see.. I’ll have to talk to my teacher about this, then. She kind of gave the impression that the weak base would completely react because the strong acid would continue to remove the OH- as a product, thereby making the reaction continue.. I’m sure I’ll get it soon. Thanks! $\endgroup$ – Quantonium Mar 16 '19 at 14:56
  • $\begingroup$ Well what would the pH be for a 0.1 molar solution of sodium acetate? It isn't going to have a pH of 7. $\endgroup$ – MaxW Mar 16 '19 at 15:33

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